Method of making zinc sulphide



May 21, 1940. I EW 2,201,522

- METHOD OF MAKING game SULPHIDE' Filed Nov. 2, 1938 Ammonium SulfateAmmonia Heat Gas Storage Ammonium Bisulfabe NH H804 water Acid ic SolidRaw Material ZnS Ore Water Suspension Gas Stor age Boil Solid SolutionZnSO (Zinc Sulfate) (NW2 4 Pure QZ 4 mert Solution o PrecipitationPurification $1804 I gg w NI- Q 80 Crystallizer Waste Gangue From Ore,Filt r Flltlate Cadmium etc. v QZ 4 Solid Crude ZnS To be washed,treated to finishing steps and calcined to pigment ZnS Finished ProductPatented May 21, 1940 PATENT OFFICE METHOD OF MAKING ZINC SULPHIDEHarlan A. Depew, Columbus, Ohio, assignor to American Zinc, Lead &smelting Company, St. Louis, Mo., a corporation of Maine ApplicationNovember 2, 1938, Serial No. 238,453

1 Claim.

This invention relates to the production of zinc sulphide pigment andhas for its object to provide a new and improved method of making N thesame.

There are two known methods in use at the present time for producingzinc sulphide from zinc sulphide ore, one of which may be called thesulphide acid process and the other of which may be called thethiosulphate process.

In the sulphuric acid process resort is had to the following reaction,to wit:

( 1) ZnS +H2SO4 TZZHSO4+H2S In this process the ore is first reactedwith lfi sulphuric acid and the hydrogen sulphide gas'is z ii' phatesolution and the zinc sulphide is precipitated. Unfortunately, however,the reactions will not go to the right at will and then to the left byjust writing them that way. Many expedients have been resorted to todrive the rezfi 'aotions to the left during precipitation, but thisreaction has not proceeded to completion-even Worse, the product is ofpoor quality. Color, hiding power and whitening strength (sometimescalled tinting strength) are relatively poor. This is probably due in nosmall part to the fact that the precipitate comes down in a finelydivided form and the particles flocculate and entrap impurities. Toobtain pigment properties by this process it is necessary to age theproduced precipitate in the liquors to grow the particles to therequired size. Moreover, the uneven size of the particles probablyaccounts for the poor hiding power and the muddy tone in tints. Theoccluded materials may partially explain the 40 poor color.

50 zinc The thiosulphate process when carried out in strict accordancewith the theory, makes a zinc sulphide that is recognized as havingsuperior properties. These superior properties are believed to be due tothe uniformly sized pigment particles of optimum hiding power. In thepractice of this thiosulphate process, the product is made by reactingammonium bisulphate (NI-I4HSO4) with zinc sulphide ore to form sulphateand ammonium thiosulphate, (NI-1028203. These materials are purified andreacted in the presence of (NH4)2S and ammonia. The resultant (NI-M2804is used to form ammonium bisulphate (NHiHSOi) to reactwith the zincsulphide ore.

' ing pigment zinc sulphide.

The process is cyclic and requires (on paper) only coal and ore plusmaterial to replace the losses and reactions due to impurities in theore. One of the difficulties in practicing this thiosulphate processlies in the fact that the reaction of the ammonium bisulphate and zincsulphide ore has to be produced by reacting in a furnace, and it isnecessary that the furnace be air tight. However, due to pastiness ofthe charge and other features, thefurnace has to be opened andthis'causes oxidation, and the ammonium thiosulphate is not alwaysformed. Moreover, there is a serious corrosion of the furnace andpossible side reactions.

By the improved process of the present application the troubles anddifiiculties experienced in both of the aforementioned processes are obviated. The practice of the invention will be readily understood byreference to the accompanying diagram from which it will appear that theprocess is cyclic. Heat is applied in any suit.- able way to ammoniumsulphate and as a result ammonia gas is given ofi and carried tostorage, leaving solid ammonium bisulphate that is acidic. This ammoniumbisulphate is dissolved in water and zinc sulphide ore is added theretoand held therein in suspension. The whole is then boiled, and thehydrogen sulphide gas given oil is carried to storage and the zincsulphide ore brought into solution, thus producing a solution of zincsulphate and ammonium sulphate. This solution, containing zinc sulphateand inert ammonium sulphate is purified, and the pure solution istransferred to a precipitation tank where the ammonia gas formed byheating the ammonium sulphate and the hydrogen sulphide gas generated onboiling the water suspension are added to the precipitation tank,resulting in the precipitation of zinc sulphide. The precipitate of zincsulphide is then filtered from the solution and may be subjected to theusual steps for mak- The filtrate containing the valuable ammoniasulphate, (NI-102804, is sent to an evaporator or crystallizer fromwhich the solid salt (NH4)2SO4 is obtained which is returned to thestarting point shown at the top of the diagram.

Experience in the old thiosulphate process heretofore referred toindicates that the reaction is as follows:

However, this reaction, since it is carried out in a furnace, occurs asa dry reaction. I have discovered, however, that when carried out inwater this reaction occurs as as indicated in the accompanying diagram.

Preferably the zinc sulphide ore, when introduced into the solution ofammonium bisulphate, should be about 100-200 mesh to be fine enough toreact at a reasonable speed and yet coarse enough to wet well. If theore is too fine it may be poorly wet and reaction will be slow.

After the zinc sulphate solution is secured by the boiling step theprocess may be varied somewhat, but in the preferred form, the zincsulphate solution is purified and then saturated with ammonia, with theresult that the zinc is in the form of a complex ammonia additioncompound. With the zinc sulphate in the solution there is a considerableamount of ammonium sulphate (NHUz S04, and I pass the hydrogen sulphidegas [evolved upon boiling ammonium bisulphate with the zinc sulphidesuspended therein, see Equation 3] into the ammoniacal zinc solution andprecipitate the zinc sulphide which is filtered, washed and. calcined bywell known methods. The ammonium sulphate in the filtrate is recoveredfor reuse in the cyclic process. The result is zinc pigment 'of superiorproperties which I ascribe to two factors, though I do not intend to bebound thereby. These two factors are l. The zinc is present as a complexion and therefore the reaction is not instantaneous as in the simpleionic reaction, and this slower reaction causes the precipitate to bemore crystalline or less colloidal.

2. The salt dissolved in water may be considered as solvated, andtherefore the solution is less water-like. As a matter of fact, a watersolution of organic salts can be made to dissolve organic material.Accordingly, through the presence of ammonium sulphate, the dissociationinto ions can be further repressed with the result that the reaction canbe slow and the product made more crystalline.

As an example of this process 100 grams of ammonium sulphate were heatedgiving oif ammonia gas and yielding 85 grams of ammonium bisulphate.This was wet with cc. of water and 30 grams of ZnS ore containing 90%ZnS was added. On boiling, hydrogen sulphide gas was copiously evolvedand carried to storage. The zinc sulphate solution was then filtered andpurified, ammonia gas was added followed by H23, and zinc sulphide in aneasily fllterable form was obtained. After calcination 25 grams of zincsulphide pigment was obtained of a high brightness, light stability andhiding power.

It may sometimes be desirable to crystallize some of the (NH4)2S04 fromthe zinc sulphate solution before precipitation, but in the generalpractice I prefer to leave the ammonium sulphate in the zinc solutionduring precipitation of the zinc sulphide.

The product of this invention has superior light resistance which Iattribute to the presence of ammonium sulphate which it is believedhinders adsorbtion of chlorides that cause light darkening. In thecyclic process it is advantageous to use additional ammonia over thatgenerated from heating ammonium sulphate, but this is not lost and canbe used over and over again.

In the precipitation of the purified zinc sulphate and ammonium sulphatesolution good results have been obtained by alternately adding thesolution and hydrogen sulphide gas although this is not the preferredpractice.

What is claimed is:

The cyclic process of making zinc sulphide which consists in heating abody of ammonium sulphate and storing the resulting ammonia gas,dissolving the resulting ammonium bisulphate in water, suspendingcomminuted zinc sulphide ore in the solution, then boiling the solutionand storing the hydrogen sulphide gas thus driven off, thus leaving asolution of zinc sulphate and inert ammonium sulphate, then introducingstored ammonia gas and hydrogen sulphide gas into the purified solution,separating the resulting zinc sulphide precipitate from the solution,then evaporating the solution and returning the solid ammonium sulphateto the starting point.

I-IARLAN A. DEPEW.

